He was born Pierre Bertin in Paris on 12 January 1899. He later changed his surname to Bernac to avoid confusion with the actor Pierre Bertin. He studied with Reinhold von Wahrlich in Salzburg. He came to music relatively late and gave his first recital in 1921.
He gave the first performances of Francis Poulenc's Chansons gaillardes in 1926. He began appearing in recital with Poulenc as his accompanist in 1934 and they continued performing together until Bernac withdrew from performing in public about 1960. Poulenc wrote a majority of his songs for Bernac, including Tel jour, telle nuit and Calligrammes. He became the most renowned interpreter of the French art song.
Bernac recorded several of Poulenc's songs, accompanied by the composer on the piano, in 1947. These were released by EMI and RCA Victor on 78-rpm discs.
When his American debut, delayed by World War II, finally came in 1948, according to Allen Hughes of the New York Times:
"the integrity and elegance of his singing quickly won him a following among connoisseurs of art song interpretation. His voice was relatively small and not naturally beautiful, but it did have a wide range, and as a result some of Poulenc's songs call for changes of register that singers other than Mr. Bernac have not always found comfortable."
He was also famous as a teacher. He taught in master classes and the American Conservatory at Fontainebleau and the École Normale de Musique de Paris. Musical Times called him "an outstanding teacher of song interpretation–visionary, precise, tireless and loving." His students include Gérard Souzay, Mattiwilda Dobbs, Elly Ameling, Jessye Norman, Bruno Laplante, Meinard Kraak, Bernard Kruysen, Margreet Honig, Michel Piquemal, Katherine Ciesinski, and Gerda Hartmann.
Bernac made a number of international concert tours with his lifelong friend Francis Poulenc as accompanist. Composers such as André Jolivet, Henri Sauguet and Jean Françaix also wrote with Bernac in mind. He also shone in German lieder and in English songs. He ceased giving public performances in 1960.
Bernac did not marry and left no survivors. He died following a series of heart attacks in Villeneuve-les-Avignon on 17 October 1979.
The Friends of the Académie Ravel in Saint-Jean-de-Luz awards the Pierre Bernac Prize in Song (Prix de chant Pierre Bernac).